1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a transflective type liquid crystal display device and method for manufacturing the same.
2. Description of the Related Art
Generally, liquid crystal display (LCD) devices are widely used for portable computers, office automation machines, and audio/video apparatuses because they are lightweight, thin, and consume low power. An LCD device includes two substrates and a liquid crystal layer interposed between the two substrates. The LCD device uses ambient light or a backlight assembly to generate light. The LCD device displays an image by controlling light transmission through the liquid crystal layer using an applied electric field to change an orientation of liquid crystal molecules. Generally, LCD devices can be classified into two categories: transmission type LCD devices and reflection type LCD devices.
FIG. 1 is a cross-sectional view of a transmission type LCD device according to a related art. Referring to FIG. 1, the transmission type LCD device includes a first substrate 102, and a second substrate 101 facing the first substrate 102.
A plurality of thin film transistors (TFTs) functioning as switching elements are formed on the first substrate 102 adjacent to crossings of gate lines and data lines. A black matrix (BM) layer, a color filter layer, and a common electrode are formed on the second substrate 101. A liquid crystal layer 103 including liquid crystals is interposed between the first and the second substrates 102 and 101. A first polarizing plate 105 and a second polarizing plate 104 are arranged on an outer surface of each of the first and the second substrates 102 and 101. An optical transmission axis of the first polarizing plate 105 has an angle of 90° relative to that of the second polarizing plate 104. A backlight assembly 106 is disposed outside the first polarizing plate 105. The backlight assembly 106 generates light and directs the generated light toward the first substrate 102.
In the related art LCD device, when the TFTs are turned on by a scanning signal applied to a plurality of gate lines, and data signals are applied to a plurality of data lines, the data signals are applied to pixel electrodes through the turned-on TFTs. A common voltage is supplied to the common electrode of the second substrate 101. Accordingly, the liquid crystals are controlled by the electric field generated between the pixel electrodes and the common electrode to transmit or block light provided from the backlight assembly 106, thus displaying a predetermined image.
The volume of the related art transmission type LCD device is large, which increases the thickness of the LCD device. Also, the backlight assembly 106 is heavy, thus increasing the thickness of the LCD device. Moreover, a power consumption of the backlight assembly 106 is excessively high.
To address these problems, a reflection type LCD device has been introduced. The reflection type LCD device uses ambient light in place of the backlight assembly 106. Such a reflection type LCD device has low power consumption. Consequently, the reflection type LCD device is widely used in portable display devices, such as electronic organizers and PDA (Personal Digital Assistant).
FIG. 2 is a cross-sectional view of a reflection type LCD device according to a related art. Referring to FIG. 2, the reflection type LCD device includes a first substrate 202 and a second substrate 201 facing the first substrate 202. A plurality of thin film transistors (TFTs) functioning as switching elements are formed on the first substrate 202 adjacent to crossings of gate lines and data lines. A black matrix (BM) layer, a color filter layer, and a common electrode are formed on the second substrate 201. A liquid crystal layer 203 including liquid crystals is interposed between the first and the second substrates 202 and 201. A first and a second polarizing plates 205 and 204 are arranged on an outer surface of each of the first and the second substrates 202 and 201. An optical transmission axis of the first polarizing plate 205 has an angle of 90° relative to that of the second polarizing plate 204. A reflector 206 is disposed outside the first polarizing plate 205. The reflector 206 reflects light provided from the ambient light of an outside and provides the same toward the first substrate 202.
In the LCD device having the foregoing structure, when a plurality of TFTs are turned on by scanning signals applied to a plurality of gate lines, and data signals are applied to a plurality of data lines, the data signals are applied to pixel electrodes through the turned-on TFTs. A common voltage is supplied to the common electrode of the second substrate 201. Accordingly, the liquid crystals are controlled by the electric field generated between the pixel electrodes and the common electrode to transmit or block ambient light reflected by the reflector 206, thereby displaying a predetermined image.
In the related art reflection type LCD device, when the intensity of ambient light is insufficient (for example, in dim light), the brightness level of the displayed image is low, and the displayed information is not readable. To resolve the above problems, a transflective type LCD device, which combines advantages of reflection type LCD devices and the transmission type LCD devices, has been proposed.
FIG. 3 is a cross-sectional view of a transflective type LCD device according to a related art. Referring to FIG. 3, the transflective type LCD device includes, a first substrate 330 and a second substrate 310, which faces the first substrate 330. A plurality of thin film transistors (TFTs) functioning as switching elements are formed on the first substrate 330 at crossing of gate lines and data lines. A black matrix (BM) layer, a color filter layer, and a common electrode are formed on the second substrate 310. A liquid crystal layer 320 including liquid crystals is interposed between the first and the second substrates 330 and 310. A first and a second polarizing plates 331 and 311 are arranged on a lower surface of the first substrate 330 and an upper surface of the second substrates 310, respectively. An optical transmission axis of the first polarizing plate 331 has an angle of 90° relative to that of the second polarizing plate 311. A backlight assembly 340 is disposed outside the first polarizing plate 331.
A plurality of pixel electrodes are formed on the first substrate 330, each connected to one of a plurality of TFTs. A passivation layer 322 and a reflector 323 are sequentially formed on the pixel electrodes. The passivation layer 322 includes a transmission hole 321, which exposes a portion (transmission region) of each of the pixel electrodes.
The region corresponding to the reflector 323 is a reflection region ‘r’. The region corresponding to the exposed portion of the pixel electrode is a transmission region ‘t’. The reflection region ‘r’ is the region within which incident ambient light is reflected in a reflection mode. The transmission region ‘t’ is the region through which light emitted from the backlight assembly 340 propagates in a transmission mode. A cell gap d1 of the transmission region ‘t’ is about twice as large as a cell gap d2 of the reflection region ‘r’ to reduce a difference in a light propagation distance between the transmission region ‘t’ and the reflection region ‘r’. Generally, a phase difference δ of a liquid crystal is obtained by the following formula:δ=Δn·d 
In the above equation, δ represents a phase difference of a liquid crystal, Δn is a refractive index of a liquid crystal, and d represents a cell gap. Thus, there is a difference in optical efficiency between the reflection mode, which uses light reflection, and the transmission mode, which uses light transmission. To reduce the difference in optical efficiency, the cell gap d1 of the transmission region ‘t’ should be greater than the cell gap d2 of the reflection region ‘r’ such that the phase difference value of the liquid crystal layer 320 is maintained constant.
Even though the difference in optical efficiency is reduced by making the cell gap d1 of the transmission region ‘t’ different from the cell gap d2 of the reflection region ‘r’, if the transmission region and the reflection region are not optimized, it is difficult to obtain optimized optical efficiency. For example, in the transmission mode, all of the light provided from the backlight assembly does not go through the transmission region. Part of the light emitted from the backlight assembly propagates within the reflection region and is not transmitted, thereby causing optical loss. Also, in the reflection mode, ambient light is not all reflected by the reflector. Instead, part of the ambient light propagates toward the backlight assembly through the transmission region, thereby causing optical loss.